Arrangement for producing a pattern on a light-sensitive surface

ABSTRACT

An arrangement for producing a pattern on a light-sensitive surface by means of a spot of light (16) capable of being moved over the surface. The spot of light is generated by means of a beam of light emitted by a lighting unit, which is directed at the light-sensitive surface and is caused to strike the surface intermittently in the form of flashes of light. This takes place whilst the beam of light is moved over the surface in accordance with specific instructions from a control unit. The pattern is made up of lines running not only in a selected reference direction, but also at right-angles to the selected reference direction, and also at at least one further fixed angle to the reference direction. The lighting unit contains an aperture by means of which the width of the line can be set. The aperture produces the spot of light (16) on the light-sensitive surface in the form of an isogonal polygon having at least six sides, with a first pair of sides (17, 21) lying opposite one another and essentially parallel to one another. A second pair of sides (19, 23) lie opposite one another, are essentially parallel to one another and are essentially perpendicular to the first-mentioned pair. At least a third pair of sides (18, 20, 22, 24) lie opposite one another, are parallel to one another and lie at an angle to the first-mentioned pair of sides at the aforementioned fixed angle. The beam of light moves for each flash of light over a certain distance which is not more than the length of those sides which extend in the direction of movement.

TECHNICAL FIELD

The present invention relates to an arrangement for producing a patternon a light-sensitive surface by means of a beam of light capable ofbeing moved over the surface.

BACKGROUND

Arrangements of the kind indicated above are usually in the form ofso-called photo-plotters, which have involved considerable investmentsuntil now. Photo-plotters are so arranged as a general rule as to becapable of being connected to CAD systems, of which various designs havebeen introduced onto the market at relatively low prices.

Technical Problem

One problem which is encounterd in this context, however, is the abilityto achieve a sufficiently low cost for the photo-plotter in relation tothe price of the more simple CAD equipment, but without jeopardizing itsviability.

The solution

The problem faced by the invention is solved by the arrangement, inwhich the spot of light is generated by means of a beam of light emittedfrom a lighting unit, which is directed at the light-sensitive surfaceand is caused to strike the surface intermittently in the form offlashes of light as the beam of light is moved over the surface inaccordance with specific instructions from a control unit, for whichpurpose the pattern is made up of lines running not only in a selectedreference direction, but also at right-angles to the selected referencedirection, and also at at least one further fixed angle to the referencedirection, in conjunction with which the lighting unit contains anaperture by means of which certain dimensions of the pattern, such asthe width of the aforementioned line, are set, and characterized in thatthe aperture is so arranged as to produce the aforementioned spot oflight on the light-sensitive surface in the form of an isogonal polygonhaving at least six sides and with a first pair of sides which lieopposite one another and are essentially parallel to one another, and asecond pair of sides which lie opposite one another, are essentiallyparallel to one another and are essentially perpendicular to thefirst-mentioned pair, and at least a third pair of sides which lieopposite one another, are parallel to one another and lie at an angle tothe first-mentioned pair of sides at the aforementioned fixed angle, inconjunction with which the beam of light is so arranged as to move foreach flash of light over a distance which is not more than the length ofthose sides which extend in the direction of movement.

Advantages

The present invention provides an arrangement which satisfies thestringent requirements in respect of reliability and precision andwhich, at the same time, can be made available at a price which is inline with the prices of the more simple CAD systems.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described below in greater detail as an illustrativeembodiment with reference to the accompanying drawings, of which

FIG. 1 shows a plan view of an arrangement according to the invention,referred to below as a photo-plotter;

FIG. 2 is a section through the arrangement along the line II--II inFIG. 1;

FIG. 3 shows a block diagram of an example of the principal functions ofthe photo-plotter;

FIG. 4 illustrates the principle embodied in the invention for producinga pattern on a light-sensitive surface;

FIG. 5 illustrates the manner in which certain areas are constructed bythe plotter;

FIGS. 6 and 7 show two embodiments of apertures contained in an opticalsystem which are used for producing the pattern on the light-sensitivesurface, and

FIG. 8 shows an example of a type of pattern, in this case a printedcircuit board for an electronics application.

BEST MODE OF CARRYING OUT THE INVENTION

The arrangement according to the invention, i.e. the photo-plotter, isessentially constructed on a rigid frame 1 made of a metal such as castiron, for example, in order to provide good dimensional stability andthe ability to remain unaffected by fluctuations in temperature. Theframe 1 supports a table 2 with a flat surface for the material which isto be provided with a pattern in accordance with the invention. Thematerial may be in the form of a light-sensitive film 15, to which thedesired pattern is applied by means of exposures. The light-sensitivefilm may be held in position in many different ways, for instance bymeans of a vacuum in the form of a number of small holes in the surfaceof the table 2, by means of which the film is held securely against thesurface through the effect of the suction in the holes.

It will be appreciated from FIGS. 1 and 2 that the table is able to movebetween the two end positions on two guides 3 in the form of rods, withthe movement being achieved by means of a motor 4 so arranged as tocause a drive shaft 5, for example a threaded spindle, for the table torotate. Mounted on the drive shaft is a carriage in the form of, forexample, a nut so arranged as to convey the table in one or otherdirection along its two guides. The nut is thus caused to move along thespindle 5 by being prevented from rotating by its connection with thetable 2. There extends over the table a bridge-like frame 6 whichexhibits two legs 7 and which supports a lighting unit 8 so arranged asto move in the direction of the arrows 9. This is achieved by thelighting unit being capable of moving along two guides 10, with themovement being achieved in a similar fashion to the movement of thetable 2, i.e. by means of an electric drive motor 11 so arranged as tocause a drive shaft to rotate, for example, a threaded spindle 12, whichinteracts with a nut or a threaded part of the lighting unit 8. Thelighting unit 8 contains an optical system 13 which contains a lightsource (not shown) and optics for refracting the beams of light emittedfrom the light source towards an aperture 14 present in the opticalsystem. Its construction is described in more detail below in relationto two ilustrative embodiments.

By locally illuminating delimited areas of the light-sensitive filmduring the relative movement of the lighting unit over selected pointson the film, it is thus possible for a pattern to be created, which, forinstance, can form the basis for the manufacture of printed circuitboards for electronics applications. The control of the movements of thelighting unit 8 and the table 2, and of the functions which determinethe appearance of the pattern is achieved in the form of control signalsfrom a control unit 70 (FIG. 3) in the form of, for instance, amicrocomputer. Program instructions for the microcomputer can beprovided either by means of a program stored in the memory or by acontrol from CAD equipment, which is used to construct the desiredpattern, in this case the printed circuit board. Control instructionsfor the various parameters can also be obtained by means of a separatecontrol panel in combination with an alpha-numerical display unit onwhich the various parameters can be read. The various controlalternatives may be provided on one and the same unit, and may beselected depending on the particular situation encountered.

The principle parameters may be appreciated from the block diagram inFIG. 3, where a block 71, which relates to the table, in this instancerepresents movement in the `y`-direction, thus the speed and position ofthe table, and a second block 72 represents movement in the`x`-direction, thus the speed and position of the lighting unit 8, athird block 73 relates to the frequency of flashing of the lightingunit, and a fourth block 74 relates to the width of the line, i.e. tothe aperture opening.

The control of the position of the lighting unit 8 relative to theposition of the light-sensitive film 15 is thus of the co-ordinatecontrol type, in which each point on the light-sensitive film can bereached through the input of control information in the `x`-directionfor the lighting unit, and in the `y`-direction for the table. Thepattern is essentially in the form of lines and dots, with the linesextending either in the `x`-direction or in the `y`-direction, or at anangle of 45° to the `x`-direction and the `y`-direction. A line in the`x`-direction is obtained by the input of control information to thelighting unit only, whilst the control information supplied to the tableis equal to zero. Movement in the `y`-direction only is achieved by theinput of control information to the table 2 only, whilst the controlinformation supplied to the lighting unit 8 is equal to zero. Movementat an angle of 45° is obtained by the simultaneous input of the samecontrol information to both the table 2 and the lighting unit 8, i.e.the same speed of movement in a certain direction. The controlinformation for all movements in actual fact contains not only speedinformation, but also directional information.

The light-sensitive film 15 is given the pre-determined pattern throughthe lighting unit 8 emitting a beam of light directed at right-angles tothe surface of the table 2, i.e the film surface, when a spot of light16 (see FIG. 4) will be produced on the surface of the film. Thelighting unit 8 is of a type in which the beam of light directed at thesurface of the film, and thus the spot of light, is not producedcontinuously, but intermittently in the form of flashes of light at acertain pre-determined frequency. The flashes of light are thus of veryshort duration at pre-determined intervals, usually 800 times per secondin this particular instance, but can be varied depending on the speed ofmovement of the lighting unit 8 and the table 2, for instance in orderto match the acceleration and retardation of these two units duringstarting and stopping in order to achieve uniform exposure. Theintermittent flashes of light which are directed at the surface of thefilm can be generated by means of, for instance, a laser or a flash lampof the xenon type, but may also be created by means of a rotating prismwhich projects the beam of light at the desired frequency towards theaperture 14, in conjunction with which the light source may be acontinuous light source, for example an incandescent lamp.

According to the invention the aperture 14 is so arranged that theintermittently generated spots of light 16 (see FIG. 4) are in the formof polygons, in the example illustrated octagons, i.e. eight-corneredareas of light, defined by eight lateral lines or sides 17-24. Theoctagons 16 are isogonal, which means that the angle between eachlateral line or side of the octagon is 135°. The opening of the aperture14 and thus the spot of light 16 also coincides with two of the sides ofthe octagon, in actual fact the opposite sides 17, 21 which extend inthe `x`-direction and two of the other sides of the octagon, in actualfact the sides 19, 23 which extend in the `y`-direction, whilst theremaining sides 18, 20, 22, 24 extend at an angle of 45° to the `x`- and`y`-directions. According to the invention the sides 17, 19, 21, 23which extend in the `x`- and `y`-directions are identical in size andare equal in length to one third of the overall width a of the octagon,whilst the remaining four sides 18, 20, 22, 24 are identical in sizewith one another, but have a length of a/3√2, i.e. a length whoseprojection onto the `x`-axis or the `y`-axis is equal to a/3.

The configuration described above of the spots of light 16 incombination with the co-ordinate control system forms the basis for thesimple method in accordance with the invention of creating a pattern oflines and dots such as is called for, for instance, by the printedcircuits used in electronics connections. The speed of movement of thespots of light 16 over the light-sensitive film 15, i.e. the speed ofmovement of the lighting unit 8 and the table 2 respectively, and thefrequency of flashing of the spots of light 16 are matched in such a waythat a flash of light wil be produced for each movement of the lightingunit 8 relative to the film 15 over a distance of a/3 in the `x`- and/or`y`-direction, i.e. the length of the sides which extend in the `x`- and`y`-directions. This distance a/3 has been calculated beforehand andentered into the permanent program of the microcomputer as the LowestSignificant Bit (LSB). The series of octagons plotted in FIG. 4 showshow the three spots of light 16 are generated consecutively whilstmoving only in the `x`-direction, i.e. with the table 2 stationary, whenthe sides 17, 21 of the octagon together constitute the longitudinaledges of a line with a width which is determined by the size of theaperture opening, i.e. the width a of the flash of light. Indicatedafter the three spots of light with a spot of light displaced upwards bythe distance a/3 in the `y`-direction is the start of a lineperpendicular to the first line and having the same width. An angle-cutouter corner of the side 22 is formed in this way, whilst the twolongitudinal edges of the line are constituted by the sides 19, 23. Theunbroken straight edge is achieved by the described matching of themoment of the flash to the distance covered, in such a way that theleft-hand corner 25, 26 of the sides 17, 21 of the octagon in asubsequent spot of light will coincide with the right-hand corner 27, 28of the preceding spot of light. An unbroken edge would be obtained evenif the distance covered were to be slightly shorter, so that a certainamount of overlap takes place, although the distance covered may notexceed the distance a/3, which would result in a `ragged` line. Thelines which are inclined at 45° are produced by the control informationbeing entered simultaneously and at identical values into both thelighting unit 8 and the table 2, i.e. for movement in both the`x`-direction and the `y`-direction, in conjunction with which the speedof movement of the lighting unit relative to the film 15 will be matchedin such a way in relation to the frequency of flashing that one exposureof the film will take place for each movement in the `x`-direction andin the `y`-direction over the distance a/3. In the case of movements atan angle of 45°, too, the frequency of flashing is controlled by thedistance covered in the `x`-direction and the `y`-direction, which isa/3, i.e. the same as in the case of movement in the `x`-direction andthe `y`-direction. This irregular octagon thus permits a very simpletype of control with digital control signals, because the controlsignals for the movement of the lighting unit and the table can be basedon the same basic quantity, in this case a/3.

The position of the octagonal spot of light in the co-ordinate system ispreferably defined in terms of the co-ordinates for the centre of theoctagon; see FIG. 6. For the sake of simplicity the length of a line isdefined as the distance between the centres of the first and the lastoctagons. The positions of the lighting unit 8 and the table 2 relativeto a reference point in the co-ordinate system are monitoredcontinuously by an optical digitizer contained, in the control unitwhich emits pulses to a position calculator for each 5 μm covered. Itsposition is compared with a flash register calculator contained in thecontrol unit with regard to the least significant bit (LSB), and whenthese two calculators are identical a flash will be emitted and the LSBfor the next position of the lighting unit relative to the table will beentered into the flash register calculator so as to permit the emissionof the subsequent flash. In this way a new flash will be emitted foreach occasion on which the lighting unit or the table has been movedover a distance equal to a/3.

In order to maintain the maximum speed whilst a line is being drawn foras long as possible before retardation beings, the co-ordinates for theend point of the line are compared continuously with the positionco-ordinates. Once the difference has reached a pre-determined minimumvalue the current to the drive motor is reduced progressively inaccordance with the microcomputer program.

An example of how symbols and shapes can be created using octagons isprovided in FIG. 6, which illustrates one possible approach to thecreation of a square or rectangular area.

Octagons equal to 1/4 of the length of side of the square or of theshort side of the rectangle are plotted in a sequence which isillustrated by means of arrows in the Figure. This produces a squarewhich is to all intents and purposes perfect, the angle-cut corners ofwhich account for only about 8% of its length of side. The same is trueof the rectangle, and in this case the percentage value relates to itsshort side. In the event of even smaller angle-cut corners beingrequired for some particular purpose when using octagons, it is possibleto use smaller octagons to fill out the corners.

The sequence begins by a movement of the spot of light upwards in FIG. 5and continues around the lateral lines of the square so as to follow thebroken line at a distance a/3 from the starting position and then tofill the remaining area as indicated by the directions of the arrows.

FIG. 6 shows an example of a first embodiment of the aperture 14 forproducing the series of octagonal spots of light. The aperture device isthus so arranged as to restrict the extent of the beam of light which isenclosed by the optical unit 13 and thus determines the external contourand width of the spots of light which are produced. The aperture ispowered by an electric motor (not shown) which is controlled from thecontrol unit for the purpose of adjusting the desired line width. Theaperture 14 comprises a toothed wheel 40 in mesh with a pinion (notshown) which is connected, if necessary via further pinions, to theshaft of the drive motor. The aperture also consists of four slides 41,42, 43, 44, each of which runs in its own guide 45. These are in theform of a recess in a plate 46 which supports the aperture mechanism.Four recesses are thus so positioned as together to form a cross whichis arranged symmetrically relative to the aperture opening 47. Eachslide exhibits an opening 48, each of which exhibits two lateral parts49, 50 which have a determining effect on the size of the commonaperture opening 47 and thus on the size of the spot of light. Otherlateral parts are positioned in such a way that they have no effect onthe aperture opening. The two lateral parts 49, 50 thus exhibit a mutualangle of 135°, in which case each section of these lateral partsconstitutes its own side of the octagon. The lateral parts are arrangedfor this purpose in a mutual position relative to one another such thatthe desired irregular octagon is produced, i.e. under the conditionsillustrated in FIG. 4. Because the four slides 41, 42, 43, 44 can beimparted with uniform linear movement in a direction respectivelytowards and away from one another, it is possible to create an apertureopening 47 which varies in size, but which at all times remains uniform.This is achieved by the toothed wheel 40 exhibiting curved and uniformlyextending guide slots 51, which extend from a radially outer position atone end to a radially inner position at the other end 53. There extendsinto each of the slots a guide pin 54 attached to each of the slides,which means that the guide pin and the slides are caused to execute amovement in a radial sense relative to the toothed wheel 40 in such away as to cause the aperture opening 47 to be varied. The referencedesignation 55 is used to denote a spring which is fitted in each recess45 in order to ensure high accuracy of the position of each slide.

FIG. 7 illustrates entirely in schematic form an alternative embodimentof the aperture arrangement 14, in which an annular toothed wheel 57 ispowered by a toothed pinion 58 on the shaft 59 of an electric drivemotor. In both embodiments the drive motor is appropriately in the formof a stepping motor, preferably with digital control, in which case acertain control signal to the control motor will produce a certainopening of the aperture without the need for complicated controlequipment. The aperture opening is determined in this case by eightslides 60 which are supported by guide pins 61, two for each slide.These guide pins also constitute shafts on which rotate the toothedpinions 63, 64 which interact with a toothed rack section 65 on eachslide. In this case only one side of each slide is defined by an edgesection 66 on the slide. This edge section 66 is so positioned inrelation to the direction of movement of the slides and the centre ofthe aperture opening 47 that the edge section is displaced with regardto its distance in a direction perpendicular to the normal of theaforementioned centre. The relative positions of the edge sections areso arranged that the different lengths of the sides of the octagon areproduced. The edge section 66 thus exhibits a pre-determined angle, moreprecisely 135°, to the direction of movement of the slide. It must bepointed out that only two of the eight slides 60 are shown in the Figurein the interests of clarity. The other slides are similarly positioned,are supported by other pins, and are driven by appropriate toothedpinions 63.

Shown in FIG. 8 is an example of a number of simple lines which havebeen produced by the exposure of the light-sensitive film and which havesubsequently been utilized in order to produce in a conventional mannerprinted circuits on printed circuit boards. In this case the exposedfilm is used after it has been developed for further processing, inwhich the exposed pattern is transferred to, for instance, a copperlaminate on an epoxy plate, said copper laminate being removed byetching at all points except those at which the pattern has beenproduced, in this way providing the conductors required for theelectronic components which will be installed one by one after theconductors have been drilled through at pre-determined points. Theconductor 29 is first provided with an enlarged end point 30 bycontrolling the aperture to the desired size after the co-ordinates forthis point have been found. This will cause a flash of light to begenerated, which, once it has moved over a distance a/3, will have beenreduced to a smaller width, i.e. a reduction of the distance a duringmovement only in the `x`-direction during successive flashes for eachmovement over a distance a/3 as far as the point of deflection 31. Atthis point control information is supplied to the table for movement inthe `y`-direction during continued movement in the `x`-direction, inwhich case the inclined section is formed with an unchanged width, inconjunction with which instructions are given prior to the final flashof light for a larger aperture opening to be provided in order to formthe end point 32. The conductor 33 is produced in a similar fashion byfinding the end point 34, by setting the aperture opening to the desiredsize, and by emitting a flash of light, whereupon the aperture openingis reduced during successive movement in the `x`-direction at aconsiderably reduced aperture opening, which is increased steeply at 67to a relatively large opening to produce a large line width. In order toarrive at a pre-determined co-ordinate, control instructions are givenfor movement in the `y`-direction only as far as an end point 36, wherethe light source is extinguished or shielded. The co-ordinate for thepoint 37 is then searched for, at which a flash of light produces a spotof light of the selected size. Control information is then supplied tothe lighting unit in order to produce movement in the `x`-direction tothe point 38, where flashes of light are emitted as a series whilstmovement in the `x`-direction continues as far as the edge 39 of thefilm. The film continues to be marked in a similar fashion with thelines and dots which are required as the basis for the particularpattern. These lines are not shown, however, in the interests ofclarity. Alternatively, the pattern may be generated by the techniqueknown as raster scanning, which is similar to the technique for theconstruction of, for instance, a television picture, in which thelighting unit follows a parallel pattern of lines in the `x`-directionwhilst moving in the `y`-direction once each line has been filled, saidmovement taking place over the distance a/3 concerned in the`y`-direction during control of the flashes of light in such a way thatthe light source is extinguished or screened above those areas at whichexposure is not to take place.

The invention is not restricted to the illustrative embodiment referredto above and described in the drawings, but may be modified in a numberof ways within the scope of the following Patent Claims. It isconceivable, for instance, to select a regular octagon, with anincreasing overlap of the spots of light relative to one another as aconsequence. It may also be possible to utilize other polygons whichhave opposite sides which are parallel to one another and other sideswhich can be used for lines situated at an angle to the lines extendingin the `x`-direction and the `y`-direction. The expression `light` isintended to denote in this context not only visible light; the beam oflight may also contain other high-frequency wave motion in the form oflight outside the visible frequency range.

We claim:
 1. An arrangement for producing a pattern on a light-sensitivesurface (15) by means of a spot of light (16) capable of being movedover the surface generated by means of a beam of light emitted by alighintg unit (8), which is directed at the light-sensitive surface andis caused to strike the surface intermittently in the form of flashes oflight as the beam of light is moved over the surface in accordance withspecific instructions from a control unit (70), for which purpose thepattern is made up of lines (29, 33) running not only in a selectedreference direction, but also at right-angles to the selected referencedirection, and also at at least one further fixed angle to the referencedirection, in conjunction with which the lighting unit contains anaperture (14) by means of which certain dimensions of the pattern, suchas the width of the aforementioned line, can be set, characterized inthat the aperture (14) is so arranged as to produce the aforementionedspot of light (16) on the light-sensitive surface (15) in the form of anisogonal polygon having at least six sides and with a first pair ofsides (17, 21) which lie opposite one another and are essentiallyparallel to one another, and a second pair of sides (19, 23) which lieopposite one another, are essentially parallel to one another and areessentially perpendicular to the first-mentioned pair, and at least athird pair of sides (18, 20, 22, 24) which lie opposite one another, areparallel to one another and lie at an angle to the first-mentioned pairof sides at the aforementioned fixed angle, in conjunction with whichthe beam of light is so arranged as to move for each flash of light overa distance which is not more than the length of each of the sides whichextend in the direction of movement.
 2. An arrangement according topatent claim 1, characterized in that the aforementioned polygon is inthe form of an octagon.
 3. An arrangement according to patent claim 1,characterized in that the octagon is irregular, in which case thefirst-mentioned two pairs of sides (17, 19, 21, 23) exhibit a lengthwhich is one third of the width (a) of the octagon, and the remainingpairs of opposite sides (18, 20, 22, 24) exhibit a length of which theprojection onto the prolongation of the first-mentioned pair of sides isthe same as their length.
 4. An arrangement according to any of thepreceding patent claims, characterized in that the movement of the beamof light over the light-sensitive surface is detected, and in that thelight unit (8) is controlled in such a way that a flash of light isemitted for each completed distance which is the same as the length ofeach of the sides of the octagon (46) which extend in the direction ofmovement.